Final answer:
Einstein's Special Theory of Relativity represents the unification of space and time into spacetime and the unification of energy and mass into the equation E = mc^2.
Step-by-step explanation:
Einstein's Special Theory of Relativity represents two great unifications: the unification of space and time into a single entity known as spacetime, and the unification of energy and mass into a single equation, E = mc^2.
The unification of space and time in Special Relativity means that the concept of simultaneity is relative, and different observers moving relative to each other may measure different times and distances for the same event. This concept is encapsulated in the famous time dilation and length contraction effects.